Ultrafiltration of solutions through microporous membrane filters is an old and well established method. Numerous types of materials ranging from animal membranes to synthetic organic polymers and many different processes of forming these microporous membranes have been described in the literature. Several reviews on ultrafiltration have been published (J. D. Ferry, Chemical Reviews, Vol. 18, (3), pp. 373-455 (1936); A. S. Michaels, Progress in Separation and Purification, Vol. 1, 297 (1968); C. J. Van Oss, Purification and Separation, Vol. 3, 97 (1972)).
Various types of ultrafilters are being used in commercial processes for performing separations and concentrations of various solutes from their respective solutions. Loeb type membranes made of cellulose diacetate are described in U.S. Pat. Nos. 3,133,132 and 3,133,137. Microporous membranes based on other types of polymeric materials including the utilization of polysulfone, cellulose butyrate, cellulose nitrate, polystyrene and other polymers are disclosed in U.S. Pat. No. 3,676,203. Other related patents cited for reference are U.S. Pat. Nos. 3,556,992, 3,579,412, 3,228,876, 3,364,288, Dutch Pat. No. 74,531 and French Pat. Nos. 555,471 and 1,075,417. Various ultrafiltration membranes used in commercial processes are marketed under the trade names of "Millipore" by Millipore Corporation, "Unipore" by Bio Rad Laboratories, "Diaflo" by Amicon Company and "Nucleopore" by Nucleopore Corporation.
When utilizing ultrafiltration membranes in such applications as the treatment of waste effluents, the separation and purification of biological fluids, the clarification of colloidal solutions and the sizing of molecules, it is desirable that the ultrafilter be resistant to compaction under pressure, insensitive to pH of the feed solution, unaffected by the solvent used and be stable at elevated temperatures. It is also advantageous that the membrane filter not be affected by repeated drying and rewetting (i.e. stable to wet-dry cycling) with the feed solution. In the quantitative separation and sizing of solute molecules of widely different molecular weights it is of utmost importance that the surface pores are of nearly the same size (i.e., isoporous) and that the size of surface pores can be varied over a wide range from a few angstroms to several thousands of angstroms. None of the membranes cited in the previous art fulfill all these requirements.
As will be hereinafter set forth in greater detail, ultrafiltration membranes prepared according to the process of this invention possess these desirable characteristics.